The Top Express.js Interview Questions You Need to Know in 2023

Express.js has become one of the most popular Node.js web application frameworks. Its minimalist approach, robust routing, and extensive middleware support make it a go-to choice for building RESTful APIs, web apps, and backend services.

As an Express.js developer, you need to demonstrate your knowledge of the framework during technical interviews. In this complete guide I will share the most important Express.js interview questions that assess your skills and experience with this framework.

Whether you are prepping for Express.js interviews as a full-stack, backend, or JavaScript developer, these questions will help you put your best foot forward and land your dream job!

A Quick Intro to Express.js

Before we get to the questions, let’s do a quick recap of what Express.js is all about:

• Express.js is a fast, minimalist web framework for Node.js. It provides helpful features like routing, middleware, view engines etc. on top of the core Node.js API.

• It simplifies building web apps and APIs by taking care of the low-level details so you can focus on your application logic.

• Express.js is unopinionated – it lets you structure your app and integrate other libraries the way you want.

• It has become the de facto standard for Node.js servers and is a key part of the MEAN and MERN stacks.

Top Express.js Interview Questions and Answers

Now let’s get right to the most popular and important Express.js interview questions that you should prepare for:

Q1. What is Express.js? How is it different from Node.js?

Express.js is a web application framework built on top of Node.js. Here are some key differences:

• Node.js is a runtime environment that lets you run JavaScript on the server. Express.js is a framework built ON TOP of Node.js.

• Node.js only provides basic web server functionality and APIs. Express.js ADDS helpful features like routing, middleware, templating etc. to make building web apps easier.

• Node.js, being a runtime, is lower level and requires you to write more boilerplate code. Express.js abstracts a lot of the common web development tasks so you can focus on your app’s unique requirements.

Q2. Explain the core concepts of Express.js including routing, middleware, and templating engines.

Some key concepts in Express.js are:

Routing – Express.js makes routing web requests to appropriate handlers extremely simple. You can define different routes that map HTTP verbs and URLs to request handler functions.

Middleware – Middleware functions are used extensively in Express.js to carry out tasks like authentication, logging, file uploads etc. Middleware can be mounted at the app level, router level or error handling level.

Templating engines – Express.js supports template engines like Pug and EJS for server-side rendering of dynamic HTML content. The view engine renders templates based on data provided to it.

Q3. How do you create a basic Express.js app to serve webpages?

// Import express const express = require('express')// Create app instanceconst app = express()// Define a route handler app.get('/', (req, res) => {  res.send('Hello World') })// Start server app.listen(3000, () => {  console.log('Server started on port 3000')})

To recap, we first import and create an Express.js instance. We define routes that map URLs to handlers. The handler sends back the response. Finally, we configure the port and start the server.

Q4. How does routing work in Express.js?

Routing refers to mapping HTTP requests to specific handler functions.

Some key points:

• Route definitions contain an HTTP method, path, and the route handler function. For example:

app.get('/users', getUsersHandler)

• Route paths can include parameters. For example:

app.get('/users/:userId', getUserHandler)

• Express.js router lets you create modular, mountable route handlers.

• route middleware can be used to carry out validation, logging etc.

• Route handlers are called in sequence depending on URL matched.

Q5. What is the importance of middleware in Express.js?

Middleware functions are key to Express.js app’s request-response cycle. Some key uses of middleware:

• Execute code before routing happens. For example, parsing requests, authentication, logging etc.

• Add extra data/methods to req and res objects for route handlers to use.

• Carry out validation and sanitization for requests.

• Handle errors and edge cases that occur in routes.

• Add post-processing capabilities to route outputs.

• Implement complex authentication/authorization at app level.

Q6. How do you implement authentication using Express.js?

Some ways to implement auth in Express.js:

• Use a Passport.js middleware for authentication strategies like OAuth or LDAP.

• For simple username/password auth, use a middleware to check credentials before allowing access to routes.

• Use the express-jwt middleware for token-based authentication.

• Store user sessions in a database/cache and verify the session id from the cookie.

• Use middleware like cors and helmet to implement security best practices.

Q7. How do you handle file uploads in Express.js?

Some ways to handle file uploads in Express.js:

• Use the multer middleware for handling multipart/form-data file uploads.

• Handle the file in a POST request route handler.

• Validate file attributes like mimetype and size before storage.

• Generate a unique filename to prevent clashes.

• Store the file on the local filesystem or cloud storage like S3.

• Return a URL for accessing the uploaded file.

Q8. How do you access POST and GET parameters in Express.js routes?

In Express.js routes, the request parameters are available in the req.params, req.query and req.body properties.

For example:

// GET /users?sort=ascreq.query.sort // POST /users { "name": "John" }  req.body.name// GET /users/234req.params.userId

So req.query contains the URL query params, req.body contains body for POST etc, and req.params contains route parameters.

Q9. How do you validate data in Express.js?

Some ways to validate data in Express.js:

• Use middleware like express-validator to validate body, params, headers etc.

• Manually validate data by type checking and sanitizing values.

• Use a schema validation library like Joi.

• Handle validation errors by returning 400 response.

• Create custom middlewares for reusable validation logic.

• Trim/sanitize values before validation.

Q10. How do you handle errors in Express.js?

Some ways to handle errors in Express.js:

• Use try-catch blocks and next(error) pattern.

• Create a centralized error handling middleware.

• Set status code accordingly – 400 for client errors, 500 for server errors.

• Log errors, stack traces on server.

• Display customized user-friendly error messages.

• Gracefully shut down the app if needed.

Q11. How do you serve static files and assets using Express.js?

The express.static middleware can be used to serve static files like images, CSS, JS files etc.

For example:

app.use('/public', express.static('assets'))// http://localhost:3000/public/logo.png

Here, the assets folder path is exposed on the /public path. The static middleware will serve any requested files from this folder.

Other ways are:

• Use a dedicated CDN and return URLs.
• Include assets directly in templates where needed.

So the static middleware cleanly handles basic static file hosting requirements.

Q12. How do you improve performance of an Express.js app?

Some ways to improve Express.